I open the door.
"Marlena!" I say, swinging the door farther open than I intend to. "What are you doing up? I mean, are you okay? Do you want to sit down?"
"No," she says. Her face is inches from mine. "I'm all right. But I'd like to speak to you for a moment. Are you alone?"
"Uh, no. Not exactly." I say, glancing back at Walter, who's shaking his head and waving his hands furiously.
"Can you come to the stateroom?" Marlena says. "It won't take but a moment."
“Yes. Of course.”
-- Sara Gruen, Water for Elephants
What does but mean? (The sentence seems like it won’t take time except that it takes only a moment, I suppose.)